Under certain operating conditions, engines that have high compression ratios, or are boosted to increase specific output, may be prone to low speed pre-ignition combustion events. The early combustion due to pre-ignition can cause very high in-cylinder pressures, and can result in combustion pressure waves similar to combustion knock, but with larger intensity. Strategies have been developed for prediction and/or early detection of pre-ignition based on engine operating conditions. Additionally, following detection, various pre-ignition mitigating steps may be taken.
For example, a fuel injection timing may be adjusted (e.g., advanced) to mitigate pre-ignition. In one example, by injecting at least some of the fuel relatively early, cylinder charge cooling may be achieved, which reduces the likelihood of abnormal cylinder combustion events. However, the early injection timing can cause the injected fuel to impinge on the cylinder walls and cause wall wetting. Fuel droplets coming off the cylinder walls can in turn generate a low octane species in the combustion mixture that acts as an ignition source for cylinder pre-ignition. Consequently, the desired pre-ignition mitigation may not be achieved.
Thus in one example, at least some of the above issues may be at least partly addressed by a method of operating an engine including a fuel injector. One example embodiment comprises, adjusting a spray angle of fuel injection to a cylinder based on an indication of pre-ignition. In this way, a spray pattern can be manipulated for each engine cylinder to reduce fuel impingement on the cylinder wall, and improve charge cooling, thereby reducing the occurrence of pre-ignition in the cylinder.
In one example, an engine control system may estimate a likelihood of cylinder pre-ignition from a cylinder pre-ignition count. Based on the cylinder pre-ignition count, a cylinder may be rich or lean injected with fuel delivered over one or more injections in a given engine cycle. The number of injections and timing of each injection in the given engine cycle may be adjusted based on the cylinder pre-ignition count. In response to an earlier injection timing (e.g., an injection timing closer to BDC), an engine controller may decrease the spray angle of fuel injected by the fuel injector to reduce cylinder wall impingement. In comparison, in response to a later injection timing (e.g., an injection timing closer to TDC), the controller may increase the spray angle to increase fuel injected onto the piston head and improve air-fuel mixing. The controller may likewise adjust a spray direction (or orientation) of the fuel injector.
In this way, the spray pattern of fuel injected in a cylinder may be adjusted to provide convective cooling without increasing the likelihood of abnormal cylinder combustion events. By adjusting the spray pattern of each engine cylinder based on respective cylinder pre-ignition counts, engine pre-ignition may be reduced even if different cylinders of the engine have different likelihoods of pre-ignition. Further, by adjusting the spray angle of fuel injection, cylinder air-fuel mixing may be improved to reduce exhaust emissions and increase power output. In this way, engine degradation due to pre-ignition can be reduced while improving engine fuel economy and exhaust emissions.
It should be understood that the summary above is provided to introduce in simplified form a selection of concepts that are further described in the detailed description. It is not meant to identify key or essential features of the claimed subject matter, the scope of which is defined uniquely by the claims that follow the detailed description. Furthermore, the claimed subject matter is not limited to implementations that solve any disadvantages noted above or in any part of this disclosure.